Eleetrieal insulator anx  method of



Patented Feb. 17, 1948 sneer-e1 nu INSULATOR mum) on Pnonudind TEE sm Berna d Horn nd lin James m thel s; nughn insla dlassi notste cdsaPlussLims i edl Bush! Englan a lir ils i co mas N Dr win Application August Serial No 5539.4 L 41:

In. .Great Nemshce invention relatesto electrical insulatprs p ared b inte s xt r in whic alumnum xi e i he m i con tituen n -itemeth, ods of making the same. Such insulators will be reierred to a 'sin e e alumi a in ul tor The are particularly suitable for use as. ins-ulatq s in sparking plugs which are employed undercondiions of. high mech n c l stress at hi h: temperatures and nder l a lity o ther al s ock. Bod. 1 made n. a ordance t t e vl v mifi are also suitabl for se f r ac -V tube thermocoup e sheaths. a d the: ike, r; wher hie-h me: c e cal tre gt a or i a yt per tures-1s es ble r e m le. t eram c nserts n Bake i sm u dinsshe ma uiae e o sintered. alum na insulatc s at t ki d des r b d i 5 e custo ary 9 emp y add t ons .1; an n o s li a o: gether with up to 5% of other oxides such as ka ne. e rthoxidss. the sinter s being ar i d out at a temperature of 1600 C. or higher. Difficulty is encountered in obtaining uniformly high density and mechanical strength in thesintered bodies. The object of the invention is. to provide sintered alumina insulators of uniformly high density and greater mechanical strength.

sintered alumina insulators are made ac; cord n to t known h s tha part of th lum na which s. not ombined- With the. silic or e as i P esent as a pha-alumina nd ha ng the ry a em n n as ensdum- The y ta ine s r tu e- Of inha inmin is br sona n axi l n ati cmhi ll, @Flfifi dens ty 3 o. -01

mth n term it alumina namely betaralu l ina, is also known, which has a difierent crystalline structure and a lower density. Beta-alumina is hexagonal. uniaxial negative, 41:1.63 to 1.65, v1.65 to 1.680 and density. 313:1. This form of alumina is sometimes found in alumina charges which have been molten and slowly cooled, or after heating mixtures of alumina containing small amounts of sodium salts. The presence of sodium oxide, certainly facilitates theformation of beta-alumina, and it has been suggested that beta-alumina is a compound or solid solution of alumina and sodium oxide containing 2.56% sodium oxide and having the formula NaAlzaOas. If alumina containing beta-alumina is heated in air at 1600 C. or higher the beta-alumina is con verted into the alpha form, whilst in a reducing atmosphere this takes place at a somewhat lower temperature. The change is believed to be due to the evaporation of sodium. Consequently in the manufacture of sintered alumina insulators where it. snou cust mary to employ: sintering emperat sesnot. less than 1.612 Q- anx be a-si ins which was termed t l wer t m er tunes is decomposed and the alpha form of aluminagnly is present. 7

We here muss that the psesence or small amounts 2 b ta elumina m alsintered nsulates resu ts n greater meshani a sat h an h sher census schtainsli a l her si t uns emneiature wnenbetaalumina can, 1. .0 lo e Y ollewms t bl ives th oi sts whl hw le name ess as athe ensity and rela: tire. i cp stren th alum na insu a ors rarer nar dlhg si tednaatrel' ou te npsr t irssamistune: conta ning 2 alias 1. tQs. and L5 7Q. H 129 t ba a ia hams ox des oi calei m nias:

irentshnomiunan miner imnu ti s Sl mmi Tamar-slurs D n 7 ew s ength ceases assess V limei ssively as: e. en er n temp ature is increased above 1400 Qa,,and -thlsprovides added mrefiectine the; smter n at a te ne tiue as: been uallr em: n xedahithemt eceolzilinasta-thenre ent l-vention a. sint red ai l i ez i sulates of: h k nd described is pm-- tested, as sinterin below the temperature at mos o th eta-alumina. is decomposed a; conta ninan ti less-than Smal -1mm um oxide, sufilcient sodium oxide sufficient only to produce from 1-10% of beta-alumina, from n to 9% of silica, with up tov5% of material se lected from the group consisting of oxides of cal clum (CaO) and magnesium (MgO). In place of these oxides of course other compounds can be used which will yield the oxides on heating, such as carbonates for example, and where the term "oxldes is used herein it is intended to include such compounds. The slntering tempersulators in order to avoid the formation oi glassy compounds our invention makes it possible to use commercial alumina containing appreciable amounts of sodium oxide, since anyalkali remaining is present as beta-alumina and not as glass.

One method of putting the invention into ef foot, as applied particularly to sparking plug insulators, will now be described by way of example. A mixture of 95 parts of commercial alumina (containing not less than 99% alumina and from 0.4 to 0.5% sodium oxide), Ziparts 01 calcium carbonate or equivalent oxide and 4 parts of a highly plastic natural aluminium silicate is ground with water until the majority of the particles have a diameter of less than 1 micron. The mass is then filtered, pressed, dried and formed to the desired shape by turning or grind aluminium oxide with a small percentage of sodium oxide sufficient only to produce from 1 to 10% of beta-alumina in the final product, /2 to 9% of silica and up to 5% of a material selected from the group consisting of the oxides of calcium (CaO) and magnesium (MgO), is finely ground and formed to any desired shape, and then sintered for a sufllcient period ata temperature well below 1600 C. to form a dense and. strong product containing an appreciable percentage of beta-alumina.

2. A method of producing sintered electrical insulators consisting essentially of alumina, wherein a mixture containing at least 90% of aluminium oxide with a small percentage of sodium oxide sufficient only to produce from 1 to 10% of beta-alumina in the final product-from :V to 9% of silica and up to 5% of a material selected from the group consisting of oxides of calcium (CaO) and magnesium (MgO), is finely ground and formed to any-desired'shape, and then sintered for some hours at a temperature in the neighbourhood of l500 C. to form a dense and strong product. 1 a 3. A method of producing sintered electrical insulators consisting essentially of alumina, wherein a mixture containing at least 90% of aluminium oxide with a small percentage of sodium oxide sufficient only to produce from 1 to 10% of beta-alumina in the final product, from /2 to 9% of silica and up to 5% of 'a material selected from the group'consisting' of oxides of 4 calcium (CaO) and magnesium (MgO), is finely ground and formed to any desired shape, and then sintered for some hours at a temperature not exceeding 1550 C. to form a dense and strong product.

4. A method of producing sintered electrical insulators consisting essentially of alumina, wherein a mixture containing at least 90% of aluminium oxide with a small percentage of sodium oxide suflicient only to produce from 1 to 10% of beta-alumina in the final product, from to 9% of silica and up to 5% of a material selected from the group consisting of oxides of calcium (CaO) and magnesium (MgO), is finely ground and. formed to any desired shape, and then sintered for some hours at a temperature and under conditions such that the final product contains corundum and an appreciable percentage of beta-alumina.

5. A method of producing sintered electrical insulators according to claim 4 wherein the aluminium oxide used in the mixture consists of commercial alumina which in itself contains a fractional percentage of sodium oxide, while the silica used in the mixture consists of natural aluminum silicate, these ingredients being ground and mixed with calcium carbonate and. the mixture sintered to form a product containing corundum, beta-alumina and mullite.

6. An article of manufacture comprising an electrical insulator of sintered alumina consisting essentially of corundum, a small proportion of mullite, and crystals of beta-alumina, and containing approximately 94% of alumina, not more than 3% of silica, not more than 3% of a material selected from the group consisting of the oxides of calcium (CaO) and magnesium (MgO), and a small percentage of sodium oxide.

BERNARD HOPPS.

COLIN JAMES SMITHELLS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

